A method of producing glass flakes is known which makes use of a spinning cup to produce a flat film of molten glass emanating radially from the rim of a rotating cup. The film is fed between two plates, forming an annular venturi and is super-cooled with forced air. The film is broken up due to the high velocity air stream and the drag (frictional resistance) imparted by it. Such a method and apparatus to perform it is the subject of EP 0 289 240.
The parameters involved in the successful production of a flat glass flake of even thickness, according to the method described in EP 0 289 240 are varied and complex. They have been found to include the following:—                glass composition, melt temperature and viscosity        temperature of glass in the melt tank        mass flow of glass leaving the tank and entering the cup        temperature of the glass entering the cup        distance between the outlet of the glass tank and entry to the cup        diameter and depth of the cup        heat dissipation properties of the cup        rotational speed of the cup        distance between the rim of the cup and entry to the radial venturi        distance between the plates forming the radial venturi        diameter of the venturi plates        volume and pressure of air being drawn between the venturi plates        temperature of the air flowing between the venturi plates        diameter and construction of the cyclone collector        
These parameters can all be varied with the result that glass flakes either are or are not produced. Flakes, if produced, may be flat or wavy. The flakes may have a substantial variation in thickness or be very consistent in thickness. The flakes may be large or small in cross-section and/or thickness.
Using the said method and with appropriate control of the above mentioned parameters, it was initially possible to prepare flake having a mean thickness range of from 1 to 10 μm. Further development work resulted in the ability to produce flake having a mean thickness of from 350 nm to 1 μm.
However such a method could not be used for the manufacture of silica (glass) flake having a mean thickness below about 350 nm.
With a view to producing flakes below about 350 nm, the apparatus disclosed in WO 2004/056716 provides for the heating of the descending glass stream from the melter tank by passing an electrical current through the said stream between an upper electrode near the exit spout from the melter tank to a lower electrical connection attached to the spinning device. However there are a number of disadvantages associated with such apparatus. Firstly, such equipment typically requires a very high voltage (typically in excess of 5,000 V and in many cases up to 10,000 V) to ensure sufficient heating and this creates a very serious risk of electrocution for the operator. Secondly, such equipment often fails to function efficiently for prolonged production runs. Without wishing to be bound by any theory, it is thought that this is because the hot glass stream entering the cup solidifies on contact with the much cooler mass of the spinning cup and forms an electrically insulating layer, which greatly reduces the amount of current that passes up the descending glass stream and thus the associated heating.
WO 2004/056716 also provides for the heating of the descending glass stream by the use of RF induction heating but that method has been found to be inefficient and difficult to implement effectively in production.